Method of melt forming a superconducting joint between superconducting tapes

ABSTRACT

Superconducting tapes having an inner laminate comprised of a parent-metal layer, a superconductive alloy layer on the parent-metal, a reactive-metal layer, and an outer laminate soldered thereon are joined in a superconducting joint by the method of this invention. The outer laminate is removed to form exposed sections, and the tapes are positioned so that the exposed sections are in contact. A melt zone within the exposed sections where the exposed sections are in contact is melted. The melt zone is at least large enough to provide sufficient parent-metal, superconductive alloy, and reactive-metal to form a melt that resolidifies as a continuous precipitate of the superconductive alloy. The melt resolidifies as a continuous precipitate of the superconductive alloy that is continuous with the superconductive alloy on the superconducting tape. Optionally, sections of the outer laminate material corresponding to the size of the exposed sections are bonded to the outermost surfaces of the joined exposed sections.

Cross reference to related application, this application is related tocopending application Ser. No. 07/561,439, filed Aug. 1 ,1990.

BACKGROUND OF THE INVENTION

This invention relates to methods of joining superconducting tapes, andmore specifically, to methods of forming a superconducting joint betweensuperconducting tapes. As used herein, the term "tape" means an elongatebody having major surfaces in the width and length dimensions, and asmall dimension, i.e., the thickness.

Superconductivity is that characteristic of certain materials whichpermits them to conduct electric currents without resistance. Asuperconducting material exhibits this characteristic only when itstemperature is below the superconducting critical temperature of thematerial and then only if it is not subject either to a magnetic fieldgreater than the superconducting critical magnetic field of the materialor to an electric current greater than the superconducting criticalcurrent of the material. Accordingly, superconductivity can be quenched,i.e., returned to a resistive state, by increasing the temperature,magnetic field, or current to which the superconducting element issubjected above the critical temperature, magnetic field, or current.Quenching of the superconductivity may occur abruptly or more graduallydepending upon the particular material, i.e., the relative breadth ofits superconducting transition state in terms of temperature, magneticfield, or current.

Superconductive bodies of laminated construction having an elongatedtape or strip configuration and the methods of producing suchsuperconductive tapes are well known. For example, British patent1,254,542 incorporated by and methods of forming the improved tapes.U.S. Pat. No. 3,537,827, incorporated by reference herein, disclosesimprovements in laminating superconductive tapes and methods forproducing the laminated tapes.

Briefly stated, it is known that selected parent-metals, either pure orpreferably containing minor alloying additions, are capable of beingreacted with other metals and forming superconducting compounds oralloys that have a high current-carrying capacity. Parent-metalsniobium, tantalum, technetium, and vanadium can be reacted or alloyedwith reactive-metals tin, aluminum, silicon, and gallium to formsuperconducting alloys, such as triniobium tin. As used herein, the term"triniobium tin" is a superconducting alloy in the form of anintermetallic compound comprised of three niobium atoms per tin atom.

Additionally, it is understood that the superconductive alloys orcompounds can be improved by first alloying the parent-metal, i.e.,niobium, tantalum, technetium, and vanadium with a minor amount of asolute metal having an atom diameter of at least 0.29 angstrom largerthan the diameter of the parent-metal atom. A broad disclosure ofvarious parent-metals, solute metals, and reactive-metals can be foundin U.S. Pat. No. 3,416,917. U.S. Pat. No. 3,429,032 discloses improvedcritical currents in triniobium tin superconducting alloy formed whenniobium containing zirconium up to about 25 percent is heated in thepresence of excess tin, and a non-metal selected from the groupconsisting of oxygen, nitrogen, and carbon.

It is also known that the reactive-metals can be alloyed to improve thesuperconductive tape. For example, the critical current density oftriniobium tin has been improved by making copper additions in thereactive-metal tin for coating on niobium tape as disclosed in,"Enhancement of the Critical Current Density in Niobium-Tin" J. S.Caslaw, Cryogenics, February 1971, pp. 57-59. As used herein, the term"reactive-metals" includes the alloys of the metals tin, aluminum,silicon, and gallium that react with parent-metals to providesuperconductive alloys, for example, a tin alloy comprised of up to 45weight percent copper.

It has been found that niobium is an important parent-metal due to thesuperior superconducting alloys which it will form. For example, smallpercentages generally greater than one-tenth weight percent of a solutemetal can be added to the niobium parent-metal to effectively increaseits current-carrying capacity. Zirconium additions are felt to be thosemost advantageous. The solute materials, for example, zirconium, areadded in amounts up to about 33 atomic percent. Other solute additivesare used in similar amounts.

The solute-bearing niobium is reacted with either tin, aluminum, oralloys thereof by contacting the niobium with either of these metals oralloys, and then heating them to an elevated temperature for a timesufficient to cause suitable reaction to occur. Especially advantageousmaterials are those of the niobium-tin compositions in which the ratioof niobium to tin approximates three to one, i.e., triniobium tin, sincethese materials have superior superconducting properties.

The triniobium tin alloy has been fabricated in various forms,particularly wires and tapes, in efforts to produce devices such as highfield superconducting electromagnets. One method for obtainingsuperconducting tape in a continuous fashion is that wherein a tape of apreselected parent-metal, such as niobium or niobium alloy, iscontinuously led through a bath of molten reactive-metal such as tin ortin alloy. The tape picks up a thin coating of the reactive-metal fromthe molten bath and the tape is subsequently heated in a reactionfurnace to cause formation of a superconductive alloy on the surface ofthe parent-metal tape.

The superconducting alloy formed on the tape is fragile, and outerlaminae of non-superconductive metal are applied to the tape to make alaminated superconductor that is strong and capable of being wound ontocoils without damage to the superconductive material. For example, arelatively thin tape of niobium foil is treated with tin to form anadherent layer of triniobium tin on the surfaces of the tape, and coppertapes of substantially the same width are soft soldered to each of themajor surfaces of the superconductive tape to form a symmetricallylaminated structure. Because of the difference in the coefficient ofthermal expansion of copper and the niobium-niobium tin material, thebrittle intermetallic compound is placed in compression even at roomtemperature, minimizing the danger of mechanical fracture when coiling.

One use for such superconductive tape is for the windings insuperconducting magnets. For example, a magnetic resonance imagingdevice can use 6 superconducting magnets, with the windings in eachmagnet requiring a continuous length of superconducting tape of over akilometer. Individual magnets in the device are connected together toprovide a continuous superconducting path through all six magnets. As aresult, a continuous length of superconducting tape of many kilometerswould be required for the device. Continuous lengths of many kilometersof superconductive tape are not currently available, and many shorterlengths would have to be joined. In addition, it can be expected thatsome breakage and damage of the tapes will occur during tape windingoperations, necessitating joints to repair such breakage or damage.

Superconducting magnets are often used in apparatus requiring a constantmagnetic field from the magnet. To maintain the constant magnetic fieldthe magnet must operate in the superconducting, or persistent mode.Current loss in the magnet from internal resistance causes drift orreduction of the magnetic field. As a result, a superconducting joint isdesirable for making the necessary connections between superconductingtapes to prevent drift of the magnetic field. The current-carryingcapacity and magnetic field behavior of the joints should at leastapproach the current-carrying capacity and magnetic field behavior ofthe superconducting tape, or the joints will become the limiting factorin the current-carrying capacity of the device.

An object of this invention is a method for forming superconductingjoints between superconducting tapes where the joints have a highcurrent-carrying capacity, approaching the current-carrying capacity ofthe superconducting tape.

Another object of the invention is a method for forming superconductingjoints between superconducting tapes where the joints sustain thesuperconductive properties in high magnetic fields approaching the highfield behavior of the superconductive tape.

BRIEF DESCRIPTION OF THE INVENTION

We have discovered a method for joining superconducting tapes to form ajoint having a high critical current capability, and high magnetic fieldbehavior, approaching the critical current and high field behavior ofthe adjoining superconducting tape. Superconducting tapes joined by themethod of this invention have an inner laminate comprised of aparent-metal layer, at least one superconductive alloy layer on theparent-metal layer, and at least one reactive-metal layer that iscapable of combining with the parent-metal and forming thesuperconductive alloy. An outer laminate of a non-superconductive metalhaving a coefficient of thermal expansion greater than that of the innerlaminate, is bonded to both sides of the inner laminate. An insulatorsuch as varnish is sometimes used to cover the outer laminate on sometapes. The parent-metal is a metal selected from the group niobium,tantalum, technetium, and vanadium.

When insulating layers are present on the superconducting tape, theinsulating layers are removed from a section of each tape, hereinreferred to as the exposed section. The exposed sections are thesections in the tapes that are joined in the method of this invention.The outer laminate is removed from the exposed sections, and the tapesare positioned so that the exposed sections are in contact. Preferably,the exposed sections are soldered together and clamped between chillplates. The chill plates cover the exposed sections except for anintended melt zone or zones. The chill plates are made from a thermallyconductive metal such as copper to conduct heat away from the melt zone.As used herein, the term "melt zone" means a preselected zone or zoneswithin the exposed sections where the exposed sections are in contact,and are to be melted.

A preselected melt zone in the exposed sections is melted andresolidifies to at least form a continuous precipitate layer of thesuperconductive alloy connecting both tapes. The melt zone is at leastlarge enough to provide sufficient parent-metal, superconductive alloy,and reactive-metal to form a melt that resolidifies as a continuousprecipitate of the superconductive alloy. A continuous layer ofsuperconductive alloy is precipitated that is continuous with thesuperconductive alloy on the superconducting tape, providing acontinuous superconducting current path between tapes. Preferably,sections of the outer laminate material corresponding to the size of theexposed sections are bonded to the outermost surfaces of the joinedexposed sections, for example, by soldering.

DETAILED DESCRIPTION OF THE INVENTION

Superconducting tapes are joined by the method of this invention to forma superconducting joint. Such superconducting joints can be used torepair superconducting tapes that are broken during winding or handling,to join short lengths of tape to form a long length of tape needed toform the winding in a large superconducting magnet, or to join separatemagnets in a series. When the joints are part of a superconductingmagnet, the superconducting properties of the joints will limit thecurrent-carrying capacity in the magnet, and as a result, limit themagnetic field that can be generated by the magnet. Therefore, thejoints should have a high current-carrying capability and a highmagnetic field behavior approaching the current-carrying capability andmagnetic field behavior of the superconducting tape.

In one embodiment of the present invention, a superconducting tape isjoined having triniobium tin as the superconductive alloy in the tape,and is herein referred to as "triniobium tin tape." Triniobium tin tapesare well known in the art being described, for example, in"Superconducting Properties of Diffusion Processed Niobium-Tin Tape," M.Benz, I.E.E.E. Transactions of Magnetics, Vol. MAG-2, No. 4, December1966, pp 760-764. Briefly described, a typical example of a triniobiumtin superconducting tape has a width of about 5 mm, and a thickness ofabout 185 microns. The tape has an inner laminate of about 33 microns,comprised of a parent-metal layer of niobium alloy of about 11 microns,superconductive alloy layers of triniobium tin of about 8 microns onboth surfaces of the niobium alloy layer, reactive-metal layers ofexcess tin alloy of about 3 microns on the superconductive alloy layers,and an outer laminate of copper of about 76 microns soldered to theinner laminate. Optionally, a varnish coating covers the outer laminateon both sides. The varnish is comprised of a mixture of equal partstoluol and menthanol mixed 4 parts to 1 with G.E. 7031 InsulatingVarnish.

The outer laminate is soldered to the inner laminate with a soldercomprised of about 37 weight percent lead and the balance tin.Optionally, the outermost surface of the copper laminate is coated withsolder to provide additional corrosion resistance for the tape. Theparent-metal is a niobium alloy comprised of up to about 5 atomicpercent zirconium, up to about 10 atomic percent oxygen, and the balanceniobium. The reactive-metal is comprised of up to about 40 atomicpercent copper with the balance substantially tin. Preferably, theparent-metal is a niobium alloy comprised of about 1 atomic percentzirconium, about 2 atomic percent oxygen, and the balance niobium, andthe reactive-metal is comprised of about 32 atomic percent copper withthe balance substantially tin.

Triniobium tin tapes that are to be joined have the varnish insulation,solder coating, and copper outer laminate removed from both surfaces ofthe tapes to form exposed sections. The varnish, solder, and copperlaminate can be removed by conventional means well known in the art. Forexample, the varnish insulation can be removed with acetone, while thesolder and copper laminate can be removed with etchants. A suitableetchant for removing the solder is comprised of ammonium bifluoride,hydrogen peroxide, and water, and is available from Cutech Inc., Pa., asCutech solder stripper SNPB 1117. Solder can be removed by dipping theintended exposed section in the solder stripper for about 30 seconds. Asuitable etchant for the copper outer laminate is comprised of 179.7grams sodium peroxodisulfate, 0.009 grams mercury(II) chloride, and 7.5milliliters phosphoric acid. The copper etchant is heated to about 50°C. and the intended exposed section is dipped in the etchant for about45 minutes to remove the outer laminate.

Alternatively, the copper outer laminate can be removed by delaminationfrom the tape, and can be reapplied to the exposed sections afterjoining. For example, a soldering iron is applied to the outer laminateto soften the solder in the area of the exposed section. The copperlaminate is then peeled away from the exposed section, and held in aposition away from the exposed section by known positioning or clampingmeans during the joining process. The delaminated sections of the outerlaminate can also be cut off and later reapplied to the outermostsurface of the joined exposed sections.

The superconducting tapes are positioned so that the exposed sectionsare in contact. It is within the area of contact between the tapes thatthe joint is formed in the melt zone. For example the tapes can bepositioned to be in the same plane, aligned in the length dimension andin abutting contact along the leading edge of the exposed sections. Theleading edges of the exposed sections are formed to be in oppositelymatching relation to form what is well known in the art as a butt joint,with the intended melt zone encompassing the abutting leading edges.Preferably, the superconducting tape is protected by chill platesclamped over the exposed sections, leaving the melt zone exposed.

With respect to positioning of the tapes, preferably, the exposedsections are overlapping so that the tapes are symmetrical in the widthdimension. The tapes are then soldered together to provide strength tothe joint during and after joining. The intended melt zone is along atleast one of the aligned edges in the width dimension of the exposedsections. In the most preferred method the melt zone is along one of theedges in the width dimension of the overlapping exposed sections.

Copper chill plates, about the width of the superconducting tape andseveral times the length of the melt zone, have a notch of predeterminedlength and depth corresponding to the size of the melt zone removed fromthe central portion of the plate, along one edge in the width dimension.The notch in the copper plates defines the limits of the intended meltzone. The copper plates are then clamped over the exposed sections sothat the notched portions of the copper plates are symmetricallyaligned, leaving only the intended melt zone exposed. The chill plateshold the superconducting tapes together and rapidly remove any excessheat during the melting operation that follows. The copper chill platescan be formed in any size or shape that rapidly removes heat from themelt zone.

The melt zone is heated in a protective atmosphere to a temperature thatmelts the superconducting tape, without vaporizing any of theconstituents of the tape. As used herein, the term "protectiveatmosphere" means an atmosphere that does not provide hydrogen or oxygenfor reaction, corrosion, or embrittlement of the tape. For example,apparatus used for tungsten inert gas welding, laser beam welding, orelectron beam welding can be controlled to melt the superconducting tapein a protective atmosphere, without vaporizing the constituents of thetape.

We have discovered that a superconducting tape, such as triniobium tin,can be melted in a melt zone and resolidified to form a continuousprecipitate of superconductive alloy that is continuous with thesuperconductive alloy on the tape adjacent the melt zone. Theresolidified melt zone is sometimes herein referred to as the "weldbead." Prior to this, it was known that superconductive alloys could beformed by heating parent-metals in the presence of reactive-metals totemperatures that cause melting of the reactive-metal and reaction withthe parent-metal, forming the superconductive alloy by a diffusionreaction. It had not been known, and it is considered very surprisingthat a parent-metal, a superconductive alloy, and excess reactive-metalcan be melted together and resolidified to form a continuous precipitateof superconductive alloy that is continuous with the parentsuperconductive alloy on a superconducting tape.

The size of the melt zone and resulting weld bead determines thecurrent-carrying capacity of the joint. Generally, it has been foundthat the current-carrying capacity of the weld bead is about 10 timesless than the current-carrying capacity of the superconductive alloy onthe tape. Therefore, to approach the current-carrying capacity in thesuperconducting tape, the superconducting cross-section of the weld beadhas to be 10 times greater than the superconducting cross-section of thetape. The superconducting cross-section is determined by means wellknown in the art, and is the width times the thickness of the continuousportion of the superconducting alloy. For example, in the most preferredjoint along one edge of the exposed sections, a preferred melt zone isat least 15 millimeters, and preferably 20 millimeters, long and 0.5millimeters wide for a 3 millimeter wide triniobium tin tape.

The melt zone reduces the cross-section of the tape and, therefore,reduces the superconducting cross-section available in the tape. Thecurrent-carrying capacity of the tape in the joint area can be reducedbelow the current-carrying capacity of the tape if the melt zone is notselected carefully. In the initial superconducting cross-section of thejoint, prior to where any current is transferred through the joint, theloss of superconducting cross-section becomes a limiting region in thetape. As a small amount of current transfers through the weld bead, thejoint is no longer limited in cross-section, having nearly twice as muchconductor as the original tape. It is, therefore, important to form aweld bead in the beginning of the joint which minimizes the loss ofsuperconducting cross-section in the parent tape.

For example, loss of superconductive cross-section in the tape can beminimized by adding additional material to the melt zone. A piece ofsuperconducting tape having the outer laminate removed is positionedover the melt zone and melted with the melt zone. The additionalmaterial adds superconducting cross-section to the weld bead withoutsacrificing the width of the superconducting tape. Another method is toform the weld bead so that it tapers out to the edge of thesuperconducting tape.

A preferred melt zone in the above described butt joint is diagonalacross the width of the exposed sections so that the weld bead is formeddiagonally across the width of the tape. This allows the current totransfer from one tape to the other at various points across the widthof the tape. If the current must transfer at a single point across thewidth, then the superconducting cross-section at this point will becritical, and must be at least the superconducting cross-section in theadjoining tapes.

The exposed sections in the superconducting tapes are at least longerthan the melt zone, and preferably are long enough to provide for easeof joint formation. After joining, a section of outer laminatecorresponding to the size of the exposed section is soldered to coverthe exposed section.

EXAMPLE I

A reel of triniobium tin tape approximately 3 mm in width was obtained.Five pairs of sample lengths of the tape, about 25 cm in length, wereremoved from the reel for joining by the method of this invention.Solder on the outermost surfaces of each tape was removed by dippingabout 5 cm of one end of each tape in the Cutech solder stripper SNPB1117, described above, for about 60 seconds. The same end sections ofeach tape were then dipped for about 45 minutes in the above describedcopper stripping solution heated to about 50° C. This removed the copperouter laminate and exposed the thin layer of solder that had bonded theouter laminate to the inner laminate forming exposed sections forjoining. About 1 cm was trimmed from the exposed sections, leavingexposed sections of about 4 cm in length.

About 12 mm of the exposed sections in each pair of sample lengths wereoverlapped so that they were symmetrically aligned in the widthdimension. The overlapping exposed sections were then soldered togetherby heating with a soldering iron. No extra solder was added because thethin layer of solder remaining on the exposed sections was sufficient tobond the exposed sections together.

Copper chill plates about 25×76×3.2 mm had a notch removed from themid-length of the plate along one edge of each plate. The notch wasabout 10 mm long and about 0.5 mm in depth with a taper at each end ofthe notch. The copper chill plates were clamped on both sides of theexposed sections so that the notches in the plates were symmetricallyaligned over the overlapping exposed sections, leaving a portion of oneedge exposed where the plates had been notched. This exposed area withinthe notched portion of the chill plates is the intended melt zone in theexposed sections.

A tungsten inert gas arc welder was used to melt the melt zone. Thetungsten electrode had a tip size of about 0.5 mm in diameter, and argonwas used as the shielding gas. An arc was drawn by touching theelectrode to the chill plate. The arc was moved from the chill plateonto the foil with a slow motion until some melting of the exposedsection occurred. After the initial melting, the arc was moved acrossthe melt zone in several slow sweeps until the entire melt zone wasmelted. After complete melting was achieved, the electrode was removedand a weld bead formed along the melt zone.

The joint was then tested to determine the current-carrying capabilityof the joint using the four probe resistance measurement technique wellknown in the art. Two voltage probes were soldered onto thesuperconducting tape a short distance from each side of the joint.Current leads were soldered onto the superconducting tape at a furtherdistance from each side of the joint. The joints were cooled to 4.2K bycooling in liquid nitrogen, followed by cooling in liquid helium. Amagnet having a magnetic field of about 5 Tesla was aligned over thejoint so that the magnetic field was perpendicular to the current pathin the superconducting tape.

A current was passed through the joint in increasing steps, and thevoltage was recorded from the probes on each side of the joint. In thistest, the critical current was defined as the current which caused avoltage differential of 0.2 microvolts between the probes. Threesections of triniobium tin tape that did not contain joints weresimilarly tested to determine the critical current of the triniobium tintape. The critical currents measured on the tape samples and joints areshown below in Table I.

                  TABLE I                                                         ______________________________________                                        Critical Current Measured at 4.2K and 5 Tesla                                             Critical Current                                                  Sample      (Amps)                                                            ______________________________________                                        Tape 1      304                                                               Tape 2      351                                                               Tape 3      362                                                               Joint 1     165                                                               Joint 2     277                                                               Joint 3     262                                                               Joint 4     208                                                               Joint 5     270                                                               ______________________________________                                    

The critical current measured in the joints made in Example 1 is atleast 50 percent of the critical current in the superconductingtriniobium tin tape. Therefore, a joint having a melt zone of about 20mm in length, about twice the length of the melt zones for the jointsmade in Example 1, should be sufficient to provide a superconductingjoint with the current-carrying capacity of the 3 mm wide triniobium tintape.

The critical current test also showed that joints formed by the methodof this invention are superconducting in a magnetic field of 5 Tesla. Amagnetic field of 5 Tesla is above the critical field of niobium, tin,and any other part of the joint except triniobium tin. Therefore, acontinuous triniobium tin current path existed through the joint inorder for the joint to be superconducting in the 5 Tesla magnetic field.

We claim:
 1. A method for joining superconducting tapes having an innerlaminate comprised of a parent-metal layer selected from the groupniobium, tantalum, technetium, and vanadium, a superconductiveintermetallic compound layer on the parent-metal layer, a reactive-metallayer that is capable of combining with the parent-metal and forming thesuperconductive intermetallic compound, and an outer laminate of anon-superconductive metal bonded to the inner laminate, the methodcomprising:removing the outer laminate from a section of separate tapesto form exposed sections, and positioning the tapes so that the exposedsections are in contact; melting the exposed sections in at least onemelt zone where the exposed sections are in contact, and resolidifyingthe melt as a continuous precipitate of the superconductiveintermetallic compound.
 2. The method of claim 1 wherein the exposedsections are positioned to be overlapping and symmetrical in the widthdimension.
 3. The method of claim 2 further comprising the step ofsoldering the exposed sections together.
 4. The method of claim 3further comprising the step of clamping chill plates on each side of theexposed sections so that a notch in each plate is symmetrically alignedover the melt zone leaving the melt zone exposed.
 5. The method of claim4 wherein the melt zone is along an edge of the exposed sections.
 6. Themethod of claim 1 further comprising the step of bonding sections ofouter laminate to cover the joined exposed sections.
 7. The method ofclaim 5 further comprising the step of bonding sections of outerlaminate to cover the joined exposed sections.
 8. The method of claim 1wherein the parent-metal layer is comprised of up to about 5 atomicpercent zirconium, up to about 10 atomic percent oxygen, and the balanceniobium, and the reactive-metal layer is comprised of up to about 40atomic percent copper and the balance tin.
 9. The method of claim 1wherein melting is performed by a tungsten inert gas electrode.
 10. Themethod of claim 5 wherein melting is performed by a tungsten inert gaselectrode.
 11. The method of claim 1 wherein melting is performed by alaser.
 12. The method of claim 3 wherein melting is performed by alaser.
 13. The method of claim 1 wherein the melt zone is at least largeenough to provide sufficient parent-metal, superconductive intermetalliccompound, and reactive-metal to form a melt that will resolidify as acontinuous precipitate of the superconductive intermetallic compound.14. A method for joining superconducting tapes having an inner laminatecomprised of a niobium layer, a superconducting triniobium tin layer onthe niobium layer, a tin layer, and an outer laminate of anon-superconductive metal bonded to the inner laminate, the methodcomprising:removing the outer laminate from a section of separate tapesto form exposed sections, and positioning the tapes so that the exposedsections are in contact; melting the exposed sections in at least onemelt zone where the exposed sections are in contact, and resolidifyingthe melt as a continuous precipitate of superconducting triniobium tin.15. The method of claim 14 wherein the exposed sections are positionedto be overlapping and symmetrical in the width dimension.
 16. The methodof claim 15 further comprising the step of soldering the exposedsections together.
 17. The method of claim 16 further comprising thestep of clamping chill plates on each side of the exposed sections sothat a notch in each plate is symmetrically aligned over the melt zoneleaving the melt zone exposed.
 18. The method of claim 1 furthercomprising the step of bonding sections of outer laminate to cover thejoined exposed sections.
 19. The method of claim 18 wherein the meltzone is along an edge of the exposed sections.
 20. The method claim 14further comprising the step of bonding sections of outer laminate tocover the joined exposed sections.
 21. The method of claim 14 whereinthe niobium layer is comprised of up to about 5 atomic percentzirconium, up to about 10 atomic percent oxygen, and the balanceniobium, and the tin layer is comprised of up to about 40 atomic percentcopper and the balance tin.
 22. The method of claim 14 wherein meltingis performed by a tungsten inert gas electrode.
 23. The method of claim19 wherein melting is performed by a tungsten inert gas electrode. 24.The method of claim 14 wherein melting is performed by a laser.
 25. Themethod of claim 19 wherein melting is performed by a laser.
 26. Themethod of claim 14 wherein the melt zone is at least large enough toprovide sufficient niobium layer, triniobium tin layer, and tin layer toform a melt that will resolidify as a continuous precipitate of thesuperconductive triniobium tin.